1. Field of the Invention
This invention relates to ion sources and, more particularly, to an ion source using a magnetic filter to improve the ion beam quality.
2. Prior Art
For producing large volumes of uniformly distributed ions with densities exceeding 10.sup.12 ions/cc, present ion sources have limited capabilities. One important application of a hydrogen ion source is in neutral-beam injection systems for fusion energy experiments and reactors. Ions from such sources are initially electrostatically accelerated to high energies and subsequently neutralized to provide beams of high-energy, neutral atoms for these neutral-beam injection systems. The neutral-beam injection systems provide megawatts of energy for heating magnetically-confined plasmas in fusion energy devices such as tokamaks and mirror fusion devices. The initially cold, or low energy, plasma ions within these fusion energy devices are heated to high energies by being bombarded with high-energy particles. It has been found that the extremely high magnetic fields of these fusion energy devices not only effectively confine the plasma but also prevent charged particles from penetrating the plasma. Because neutral particles are not affected by magnetic fields, high-energy neutral particles provide a good choice for heating these fusion plasmas. When energetic neutral particles or atoms enter the fusion plasma, they are re-ionized by the plasma electrons. These energetic or hot ions are then contained by the reactor magnetic fields.
One important requirement for an ion source to be used in this type of application is that the ion source should have good beam quality, that is, generate a dense, uniformly intense, stable ion stream. Another important requirement for an ion source used in neutral-beam applications is that only certain desired ion species should be produced. For example, for a hydrogen ion source, it is important that as many of the H.sup.+ ion species as possible are produced and that as few of the heavier H.sub.2.sup.+ and H.sub.3.sup.+ ion species as possible are produced. Positive hydrogen ions from a hydrogen ion source are first electrostatically accelerated to high energies. The high-energy positive ions are subsequently neutralized by being passed through a low-pressure gas cell where charge exchange neutralization takes place. When an H.sup.+ ion is neutralized by charge exchange, it becomes a neutral H atom with the same energy as the H.sup.+ ion. However, when a H.sub.2.sup.+ ion is neutralized by charge exchange, it becomes two neutral H atoms, each H neutral atom having one-half of the energy of the original H.sub.2.sup.+ ion. Similarly, an H.sub.3.sup.+ ion has one-third of the energy of an original H.sub.3.sup.+ ion. Less energetic neutral H atoms do not penetrate far enough into the plasma before being re-ionized. If they are re-ionized at the edge of the plasma they are thrown back out of the plasma to hit the container wall and cause wall damage as well as provide a source of unwanted impurities. It should be apparent that the overall efficiency and operating cost of a neutral-beam injection systems is increased by improving the percentage of desired ion species delivered by an ion source.
A typical source for producing positive hydrogen ions is shown in U.S. Pat. No. 4,140,943, granted Feb. 20, 1979 to Kenneth W. Ehlers. Hydrogen is injected into a plasma generator vessel where it is ionized by a high-current discharge provided by a plurality of tungsten filaments.